1. Field of the Invention
The present invention relates to improved yellow iron oxide pigments and to their use.
2. Brief Description of the Prior Art
The precipitation process and also the Penniman-Zoph process of producing yellow iron oxide pigments have been known for a long time. The typical course of these processes is described by way of example in Ullmann's Encyclopaedia of Industrial Chemistry, 5th Ed., Vol. A20, p. 297 ff., or in U.S. Pat. No. 1,327,061 A1 and U.S. Pat. No. 1,368,748 A1 and DE 3907910 A1. The raw material and electrolyte catalyst used is commonly iron(II) sulphate, which is obtained in the pickling of steel sheets or in the production of titanium dioxide by the sulphate process. A further possibility is the use of iron (II) sulphate from the production of low-alloy steel residues in the form of sheets, wire rolls, Fe powder with sulphuric acid. In general, α-FeOOH (yellow iron oxide) is prepared from iron (II) salts by the precipitation process (DE 2 455 158 A1) or by the Penniman process (U.S. Pat. No. 1,368,748 A1, U.S. Pat. No. 1,327,061 A1). In both processes a nucleus is produced first of all, onto which then, in a further step, relatively slowly, additional α-FeOOH is caused to grow.
The yellow iron oxide pigments produced by these processes are notable for a bright, yellow colour, but have an oil absorption value the end user finds unfavourable.
The oil absorption value is determined in accordance with DIN 53199, which dates from 1973. With the aid of the oil absorption value the practitioner is able to estimate the binder demand of a pigment. The higher the oil absorption value, the higher, too, the binder demand. The oil absorption value also, moreover, permits conclusions concerning the level of the pigment-volume concentration to be expected.
A pigment with a lower oil absorption value can be processed in a more eco-friendly manner, thereby simplifying the process and lowering the energy consumption for the end user. Low oil absorption in oxides allows the user, especially in the paint industry, to use a composition having
a greater amount of oxide and a lower level of additives in order to achieve the desired quality for the product. The higher pigment-volume concentration permits a greater colour density in paste production, which in turn is beneficial to the specific transport costs of the pigment pastes.
A high colour strength additionally boosts the desired effects for the end user.
It was an object of the present invention, therefore, to develop pigments which, with a low oil absorption value and high colour strength, have a colour locus comparable with that of prior-art pigments.